Asset Impairment and Restructuring - Narrative (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2020 USD ($) asset_group	Mar. 31, 2020 USD ($) asset_group	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 33,709,000
Restructuring expense	$ 22,435,000		$ 0	$ 22,435,000	$ 0
Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment			$ 5,826,000		$ 5,826,000
Restructuring expense	22,435,000			22,435,000
Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	161,738,000
All Other
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	5,000
All Other | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Impaired Long-Lived Assets Held and Used [Line Items]
Restructuring expense	$ 1,993,000			1,993,000
CAPP - Met
Impaired Long-Lived Assets Held and Used [Line Items]
Number of assets impaired | asset_group	3	4
CAPP - Met | Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 0	$ 32,951,000
CAPP - Thermal
Impaired Long-Lived Assets Held and Used [Line Items]
Number of assets impaired | asset_group	4	3
CAPP - Thermal | Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 17,385,000	$ 758,000
CAPP - Thermal | Operating Segments | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Impaired Long-Lived Assets Held and Used [Line Items]
Restructuring expense	2,310,000			2,310,000
NAPP | Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	144,348,000
NAPP | Operating Segments | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Impaired Long-Lived Assets Held and Used [Line Items]
Restructuring expense	18,132,000			$ 18,132,000
Mineral rights
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	18,605,000	21,825,000
Property, plant and equipment
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	134,480,000	6,066,000
Acquired mine permits, net
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 8,653,000	$ 5,818,000